1. Field of the Invention
The present invention relates to an optical pickup system for writing/reading out information on/from a magneto-optical disc, and more particularly to an optical pickup system having a simplified structure capable of improving read-out speed and reducing manufacturing cost.
2. Description of the Prior Art
FIG. 1 illustrates a construction of a conventional optical pickup system.
Referring to FIG. 1, the optical pickup system includes an optical disc 7, a semiconductor laser 1 used as a light source, and a diffraction grating 2 for forcing laser beam from the semiconductor laser 1 to be one main beam and two sub-beams for detecting a tracking error of the disc 7. A collimator lens 3 allows the three beams passed through the diffraction grating 2 to be parallel beams. With respect to the parallel beams having passed through the collimator lens 3 or beams reflected by the optical disc 7, all S-polarized wave and some P-polarized wave are reflected by a polarizing beam splitter (hereinafter simply referred to as "PBS") 4 which transmits the other P-polarized wave. Also, a reflection mirror 5 reflects P-polarized three beams L1, L2 and L3 passed through the PBS 4 toward optical disc 7, and an objective lens 6 focuses the P-polarized three beams L1, L2 and L3 reflected by the reflection mirror 5 onto the disc 7 or forces three beams L1, L2 and L3 respectively mixed with P-wave and S-wave reflected by the disc 7 to be parallel beams. In addition to these, the optical pickup system includes a modified Wollaston prism 8 which separates the parallel beams mixed with the P-wave and S-wave incident from the disc 7 via the PBS 4 into P-wave, S-wave and (P+S)-wave for the purpose of separating the incident three beams into five beams, an image-forming lens 9 for imaging the five beams separated via the Wollaston prism 8, a concave lens 10, and a photodetector 11 divided-by-eight.
The concave lens 10 has a toric surface which induces astigmatism to the main beam having passed through the image-forming lens 9 in order to detect a focus error. The photodetector 11 divided-by-eight is constructed as shown in FIG. 2, in which sections a, b, c and d in the center of the divided-eight area are for focusing the beam of (S+P)-wave component separated from the main beam incident from the concave lens 10, so that the focus error is monitored by a signal detected in the sections a, b, c and d. Reference symbols e and f on the upper and lower portions of the sections a, b, c and d designate sections for focusing the beam of P-wave and S-wave components respectively separated from the sub-beams, so that the tracking error is monitored by means of a signal difference detected in the sections e and f. Sections i and j on the left and right portions of the sections a, b, c and d are for focusing the beam of S-wave and P-wave components separated from the main beam, so that the existence of information on the disc 7 is determined by means of a signal detected in the sections i and j.
In the conventional optical pickup system, the beam radiated from the semiconductor laser 1 is transformed into one main beam L1 and two sub-beams L2 and L3 via the diffraction grating 2, and S-wave of the three beams passed through the diffraction grating 2 is reflected to allow only the P-wave to advance toward the disc 7 via the collimator lens 3, PBS 4, reflection mirror 5 and objective lens 6, sequentially. Furthermore, the three beams L1 to L3 reflected by the disc 7, which respectively mixed with P-wave and S-wave, are incident to the Wollaston prism 8 via the objective lens 6, reflection mirror 5 and PBS 4, and then separated into P-wave, S-wave and (P+S)-wave via the Wollaston prism 8. Thus, the separated beams are supplied to the photodetector 11 via the concave lens 10.
The operation of the foregoing optical pickup system will be described in detail with reference to FIGS. 2 to 6.
The laser beam emitted from the semiconductor laser 1 being the light source is diffracted into one main beam L1 and two sub-beams L2 and L3 via the diffraction grating 2, and the main beam L1 and sub-beams L2 and L3 are changed into the parallel beams by the collimator lens 3 to be incident to the PBS 4. With respect to the three beams L1 to L3, the PBS 4 reflects 100% of S-wave and 50% of P-wave, and transmits the other 50% of P-wave. In other words, because the S-wave is fully reflected and P-wave is partially reflected by the PBS 4, just three beams L1 to L3 that are P-polarized via the PBS 4 are transmitted to be incident to the reflection mirror 5 which then reflects the incident three beams toward the disc 7. The reflected beams are focusing onto the disc 7 through the objective lens 6.
The three beams L1 to L3 are focusing onto the disc 7 by means of the objective lens 6 as shown in FIG. 3, in which the main beam L1 is utilized for reading out information and detecting the focus error, and two sub-beams L2 and L3 are utilized for detecting the tracking error. The three beams are reflected from the disc 7 with holding both information (i.e., pit information or kerr rotation by a magnetization direction) written on the disc 7 and information required for detecting the tracking error.
At this time, only the P-polarized beam is focusing onto the disc 7, but the parallel beams reflected from the disc 7 differ in view of the information whether the information is written on the disc 7 or not. In more detail, when the information is not written on the disc 7, the parallel beams reflected from the disc 7 include only the P-wave component without containing the S-wave component. Whereas, the parallel beams from the disc 7 are mixed beams having S-wave and P-wave components when the information is written on the disc 7. The parallel beams reflected from the disc 7 via the objective lens 6 is reflected by the reflection mirror 6 to be incident to the PBS 4 which, with respect to the incident parallel beams, reflects all S-wave and 50% of the P-wave, and transmits the other 50% of the P-wave. Therefore, of the parallel beams reflected by the disc 7, the S-wave component is fully reflected through the PBS 4 to be incident to the Wollaston prism 8, and the P-wave component is transmitted and reflected in halves via the PBS 4 to be incident to the Wollaston prism 8.
As shown in FIG. 4, the Wallaston prism 8 separates the main beam into three beams by separating the main beam L1 in the parallel beams incident via the PBS 4 into the S-wave, P-wave and (P+S)-wave, and separates the sub-beams L2 and L3 into the P-wave and S-wave, respectively. By this operation, the parallel beams incident from the PBS 4 are separated into five beams through the Wollaston prism 8 to be incident to the concave lens 10 via the image-forming lens 9.
The concave lens 10 having the toric surface to induce astigmatism increases an angle between five beams incident from the image-forming lens 9 while producing the astigmatism in the main beam to detect the focus error. The five beams passed through the concave lens 10 are focusing into the photodetector 11 divided-by-eight as shown in FIG. 5.
The tracking error and focus error as well as the information written on the disc 7 are monitored in accordance with the shapes of the five beams focusing onto respective sections of the photodetector 11 divided-by-eight.
Here, the tracking error by means of the sub-beams is monitored as defined in equation (1), in which the signal difference of the sections e and f of the photodetector 11 divided-by-eight is detected as a tracking error signal TES, using a three-beam monitoring method: EQU TES=Se-Sf (1)
where Se and Sf denote signals in the sections e and f of the photodetector 11 divided-by-eight.
As illustrated in FIG. 6, along with differing a distance between the disc 7 and objective lens 6, the beam that is focusing onto the sections a, b, c and d of the photodetector 11 divided-by-eight is varied. At this time, FIG. 6A illustrates the focusing pattern of the beam in the sections a, b, c and d when the objective lens 6 and disc 7 are reasonably distanced not to cause the focus error, FIG. 6B is the focusing pattern of the beam in the sections a, b, c and d when the objective lens 6 is distant from the disc 7 to cause the focus error, and FIG. 6C is the focusing pattern of the beam in the sections a, b, c and d when the objective lens 6 is near to the disc 7 to cause the focus error.
A focus error signal is monitored by the signal difference according to the variation of the beams in the sections a, b, c and d, which is expressed by the following equation (2): EQU FES=(Sa+Sc)-(Sb+Sd) (2)
where, FES designates the focus error signal, Sa, Sc, Sb and Sd are signals in the sections a, c, b, and d of the photodetector 11 divided-by-eight, respectively.
As can be noted in equations (1) and (2), if there is no tracking error and focus error, the tracking error signal TES becomes zero and the focus error signal FES becomes zero, too.
The information written on the disc 7 can be monitored by the beam that is focusing onto the sections i and j of the photodetector 11 divided-by-eight by meas of the S-wave and P-wave components separated from the main beam. When the magneto-optical signal (kerr rotation by the magnetization direction) is read out, the information is monitored by a difference between signals Si and Sj in the sections i and j, as is defined in equation (3) below: EQU Optical information signal (magneto-optical signal) =Si-Sj (3)
On the other hand, the pit signal of uneven shape written on the disc 7 is monitored by the variation of light quantity in the sections i and j of the photodetector 11 divided-by-eight as below: EQU Optical information signal (pit signal) =Si+Sj (4)
However, the conventional optical pickup system heretofore has the following drawbacks.
That is, the diffraction grating for forming the sub-beams from the laser beam of the semiconductor laser must be employed to monitor the tracking error by means of the three beams. Moreover, the astigmatism is utilized for detecting the focus error by the three beams to require the toric concave lens that is difficult to be fabricated and is expensive. In addition to the concave lens, the Wollaston prism which is also difficult to be fabricated is used for separating the mixed beam having the P-wave and S-wave for detecting the focus error from the main beam reflected by the disc.
Furthermore, for allowing the beam emitted from the semiconductor laser to be partially incident to the disc or the beam reflected by the disc to be partially incident to the Wollaston prism, the pentagonal PBS which leads to fastidious fabrication is fastidious is used.
Briefly, in the conventional optical pickup system, so many optical parts are used for reading out information written on the magneto-optical disc, which incur high expense and are difficult to be fabricated. Thus, the structure of the optical pickup system is complicated while increasing the manufacturing cost. Additionally, the great number of optical parts increases the inherent weight of the optical pickup system to lengthen access time for reading out the information written on the disc, resulting in lowering the information read-out speed.